Conventionally, LSUs for scanning with a laser beam or an LED array with LEDs disposed in one line are commonly used as exposure devices for creating an electrostatic latent image in a photosensitive material. An LSU requires a polygon mirror rotated at tens of thousands of revolutions per minute (rpm), has a long optical path and requires a large number of optical components such as a lens. Accordingly, it is difficult to produce LSUs of smaller size and to adapt them to be operated at still higher speeds.
An LED array is generally constructed of a substrate of a III-V group compound semiconductor such as GaAs, resulting in high cost of material. Further, it requires a technique of precisely arranging a plurality of LED chips each having a plurality of light emissive elements, and also requires a drive circuit chip on a single-crystal silicon substrate to be connected to LED chips of GaAs using wire bonding, making it more difficult to reduce the cost.
Since higher resolution requires emissive elements to be integrated more densely, wire bonding constraining interconnection with the driver IC from being made more densely is particularly problematic. One known solution is “time division driving,” which divides one line of LEDs into eight blocks, for example, to provide eight emissions shifted along the time axis. This will advantageously relieve the density of interconnection between the densely disposed emissive elements and the driver IC, alleviating load due to wire bonding.
Specifically, when light is produced by 64 emissive elements with 20 micron pitch, 8 block time division driving with an interconnection in a matrix can reduce the number of lines connecting with the driver IC to 16 (8+8=16) and reduce the connection pitch to 80 microns, which is 4 times the pitch of each emissive element (64/16=4).
However, in the above example with 8 elements in one block, the required amount of light needs to be obtained in a time period 8 times smaller than is the case when time division driving is not performed, requiring more amount of light (emission intensity per unit of time) of an emissive element. Specifically, the required amount of light is 8 times larger than is the case when time division driving is not performed. Further, time division driving requires image data to be rearranged, thereby increasing the scale of circuitry.
Consequently, LED arrays, while smaller than LSUs and thus significantly more advantageous in size, are disadvantageous compared to LSUs in terms of cost and performance and thus have not gained popularity.
Instead of using emission principles of LEDs, an exposure device employing inorganic ELs is disclosed in the Journal of the Society of Electrophotography of Japan, Vol. 30, No. 4, 1991.
Besides these exposure devices, performances of organic ELs have been significantly improved in recent years, leading to ongoing considerations of putting these devices to use in display applications. Since organic ELs are employed with displays, the substrate is generally a transmissive glass or resin substrate, although the use of a single-crystal silicon substrate is disclosed in Japanese Patent Laying-Open No. 9-114398. It discloses the use of a single-crystal silicon substrate to provide a smaller matrix configuration of driving devices and a greater aperture efficiency in surface emission, the ability to prevent degradation due to thermal fatigue, and the like.
An exposure device employing such inorganic ELs, however, requires an alternating-current high voltage pulse with 250 volts for driving the device and has a low response rate at several hundreds of μsec. and other problems, which have hampered its commercialization.
Also, when an organic EL of the surface emission type for use with displays is to be applied in an exposure head of a printer, one serious problem arises as to how to provide the required amount of light for illuminating a photosensitive material.
For example, assuming the sensitivity of a common organic photosensitive material, E, to be 0.5 [μJ/cm2], the process rate V to be 120 [mm/s] and the resolution R to be 600 [dpi], then the required energy on the surface of the photosensitive material, W, is generally calculated using the following equations: when the assumed values provided above are substituted into the equation: W=E/(25.4/W/V), a representation in SI notation is: W=14 [W/m2].
Further, the organic EL of the surface emission type is characterized by a large angle of radiation, which is advantageous for a display due to a larger angle of field, but causes a problem for an exposure head of a printer because, in an exposure head that requires image optics, a larger angle of radiation results in less efficient use of light in the optics.
Supposing the efficiency of use of light in optics to be at 10%, the required amount of light from a light source is 140 [W/m2]. For a resolution of 1200 dpi, the required amount of light is two times larger. Providing this amount of light using an inorganic EL is extremely difficult without compromising the lifetime of the organic EL.
Another problem occurs in conjunction with imaging optics. Specifically, when a device using an array of emissive elements such as LEDs is employed for an exposure head of a printer, the optics generally have a lateral magnification of one time, as in a rod lens array. When printing on an A3 paper, for example, the required width of an image surface corresponds to the width of an A3 paper i.e. approximately 300 mm, so that an array of emissive elements may have a width of about 300 mm for optics with a lateral magnification of one time.
In the case of magnifying or reducing optics, the load on imaging optics is increased for removing aberration due to a larger angle of view, so that providing a smaller size is difficult. In addition, reducing optics has a width of an array of emissive elements larger than 300 mm.
When using imaging optics with a lateral magnification of one time such as a rod lens array, the size of an image spot is larger than that of the source due to aberrations of a lens or MTF degradation. The required size of an image spot ranges from about 60 to 80 microns for a resolution of 600 dpi, and ranges from about 30 to 50 microns for 1200 dpi. The size of an emitting portion of an LED source is several microns and therefore may be considered as a point source, which results in a smaller load on the imaging optics, realizing the above size.
On the other hand, in the case of an organic EL of the surface emission type, increasing the emitting area to compensate for an insufficient amount of light as mentioned above results in a correspondingly increased size of the source (i.e. its emitting area). In other words, for an organic EL of the surface emission type, there is a trade-off between the increase in the amount of light and the load upon the imaging optics. Consequently, it is theoretically impossible to provide an emitting surface that is larger than the size of the required image spot for optics with a lateral magnification of one time.